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Stock Based Compensation (Tables)
 3 Months Ended
Mar. 31, 2020
Share-based Payment Arrangement [Abstract]  
Schedule of stock based compensation expense
The Company recorded stock-based compensation expense in the following expense categories of its accompanying consolidated statements of operations and comprehensive loss for the three months ended March 31, 2020 and 2019, respectively (amounts in thousands):
Three Months Ended
March 31,
20202019
Research and development
$300  $423  
General and administrative780  516  
Total
$1,080  $939  
Summary of the activity related to stock option grants to employees and nonemployees
The following table summarizes the activity related to stock option grants to employees and nonemployees for the three months ended March 31, 2020:
Shares
Weighted
average
exercise price
per share		Weighted-average
remaining
contractual
life (years)
Outstanding at December 31, 20192,498,606  $17.18  7.7
Granted857,875  7.72  
Forfeited(15,749) 14.65  
Outstanding at March 31, 20203,340,732  $14.76  8.0
Vested and exercisable at March 31, 20201,230,674  $23.08  5.8
Vested and expected to vest at March 31, 20203,340,732  $14.76  8.0
Summary of grant date fair value assumptions The grant date fair value of each option grant was estimated during the three months ended March 31, 2020 and 2019 using the following assumptions within the Black-Scholes option-pricing model:
Three Months Ended
March 31, 2020		Three Months Ended
March 31, 2019
Expected term (in years)6.086.05
Expected volatility78 %80 %
Risk-free interest rate1.36 %2.55 %
Expected dividend yield0 %0 %